Devices for sensing the liquid level in tanks subjected to varying accelerations have been used heretofore, for instance, in motor vehicles for monitoring the liquid level in tanks containing hydraulic fluids, lubricating oils, cooling liquids and fuels.
One typical example is the monitoring of the level of the lubricating oil used in internal combustion engines. At present, there are two different methods available by which the level of the lubricating oil can be monitored.
In the case of the so-called "static warning" method, a one-time oil level measurement takes place while the vehicle is still at rest, especially prior to starting the engine. For instance, the liquid level is measured at the time the ignition is turned on. For this purpose, a great variety of suitable sensors are available, including optical sensors, resistance-type elements, thermo elements, etc. If the oil level is too low, a warning indicating means, such as a warning lamp, is activated after the engine has been started.
The disadvantage of this method is that the same sensor cannot be used to detect an oil shortage that might occur during vehicle travel, because the oil level in the crankcase is lower when the engine is running than when it is not running, and the oil level varies during vehicle operation.
The second method, while falling into the same general category of liquid level sensing systems, is based on what is called the "dynamic warning" principle. Here, the oil level is measured only when the engine is running, i.e., when it is hot, and any oil shortage occurring during vehicle travel is indicated. One type of sensor that is being used in this system is a float switch which is arranged at a suitable place in the crankcase. The float switch may include a reed contact which is contained in a damping cup. However, a liquid level sensor of this type may, under certain vehicle operating conditions, simulate an oil shortage condition which, in fact, does not exist. This must not be allowed to happen. For instance, if the body of lubricating oil, due to acceleration and weight forces generated as a result of vehicle cornering, changes in speed or travel on inclined road surfaces, changes its position in the crankcase, the oil level at the place where the oil level sensor is located may be lower than the oil level which would exist if the vehicle were to travel at a constant speed on a level road, or if the vehicle were at rest. It is for this reason that in the case of the "dynamic warning" method, the warning indicating means, i.e., the warning lamp, is not actuated unless the sensor signal has been applied thereto by the sensor uninterruptedly for a specified period of time. After being actuated, the warning indicating means may remain in the "warning" mode, or may be turned off after a specified period of time. The disadvantage of this prior art liquid level sensing method is that no level sensing can take place, and therefore no warning be issued, prior to vehicle travel, because if, at the start of vehicle travel, a low oil level signal is issued by the sensor, it will not be on long enough to actuate the warning signal indicated means. Another disadvantage of this prior art method is the difficulty of finding a suitable installation location for the level sensor, and to decide on a suitable minimum time interval during which the sensor signal must be on to activate the warning signal. The problem associated with determining a suitable minimum time interval is that a compromise must be made to satisfy two conflicting requirements. On the one hand, since allowance must be made for those instances where the liquid in the reservoir is subjected to accelerations and skewing for a relatively long period of time, it is desirable that, if false alarms are to be avoided, the time interval be relatively long. However, in those situations where the oil level is below the threshold value during a period of time when the body of oil in the crankcase is not subjected to accelerations, i.e., when the vehicle travels at a constant speed on a level road, or when the vehicle is not moving at all, it is desirable that, in order to avoid engine damage, the time interval be relatively short, since there exists a situation where the oil level has not just seemed to drop, but has actually dropped below the specified threshold value.